The present invention relates to racks for supporting workpieces in industrial coating operations and, more particularly, to such racks having removable hooks for hanging such workpieces. It finds particular application to industrial electrostatic coating systems.
Industrial coating systems employ a conveyor from which workpieces are hung on hooks from an overhead conveyor and are conveyed through several stations, usually including stations for performing the steps of cleaning, rinsing, drying, and coating the workpieces, and thereafter baking the coating. In many industrial coating operations, the workpieces travel through an electrostatic coating booth wherein the electrically grounded workpieces are sprayed or coated with electrically charged coating particulates, either liquid or powder. After coating and baking, the coated workpieces are removed from the hooks, and the hooks and supporting racks are reused in another coating cycle. Because of the recycling of the hooks and racks, they become coated and encrusted with multiple layers of the coating material.
It is important in those instances where electrostatic coating is employed that the support rack be electrically conductive so that workpieces can be maintained in a grounded state. The workpieces are electrically connected to ground through a conductive support rack so that electrically charged particulates are attracted to the workpieces by the electrostatic field. Thus, it is desirable to provide a support rack wherein the electrical contact between individual hooks depending from the rack are maintained in low-resistance electrical contact with the rack so that the workpieces suspended from the hooks will be satisfactorily grounded.
Each coating cycle applies a coating layer to the exposed, workpiece-engaging portion of the support hook, which coating covers the entire surface of the hook except for the small area where the hook is in contact with the workpiece. Unless identical workpieces are coated in subsequent cycles, so that the workpiece is in electrical contact with the hook at an identical position, it becomes necessary to clean the support hook to assure the requisite electrical contact between the workpiece and the support hook. Accordingly, it is desirable that the support hooks be detachably engaged with the rack so that coated hooks can be periodically removed and replaced with new or cleaned hooks. Simple and rapid interchangeability of the support hooks is also desirable so that different sizes and shapes of support hooks can be interchanged as desired so that the rack may be used for coating a wide variety of workpieces. It is desirable to provide this interchangeability while still maintaining good electrical contact between each support hook and its respective rack during coating operations.
In this connection, it has been found desirable to form a support hook in the shape of a spring to bias a clean surface of the hook against a clean surface of its support rack. An early example of such a cooperative hook and rack construction may be found in U.S. Pat. No. 1,533,805, relating to a rack for electroplating workpieces. A more recent example of such a cooperative hook and rack construction may be found in U.S. Pat. No. 5,531,334, for electrostatically coating workpieces. A substantially improved system is disclosed in my copending application Ser. No. 455,150.
Prior art hook and rack constructions have been found to have certain shortcomings in industrial coating operations, and it is the principal object of the present invention to provide a hook and rack construction having substantially improved performance.
In accordance with the present invention, there is provided an inexpensive support rack and cooperating hook for supporting workpieces during transport through the various stations of an industrial coating operation. The rack comprises a horizontal bar or beam having a first flange, a second flange, and a web joining the flanges. The first flange has a hook-receiving aperture formed in it, and the second flange has a hook-receiving slot formed in it extending inwardly from an edge of the flange web and thence outwardly toward the edge to thereby provide a detent. The opening and the slot are aligned to receive a hook from which a workpiece may be hung. The hook is bent to form a spring having a terminal leg and a depending body leg. The terminal leg extends into the aperture in the first flange, and the depending leg extends through the slot formed in said second flange behind the detent. The web is formed so as to provide a fulcrum against which said spring bears. The force of the spring against the fulcrum biases the dependent leg behind the detent and against the blind end of the slot.
The bar and hook in combination provide means by which the hook may quickly be installed in the bar. The spring is biased against the side of the opening in the first flange and against the blind end of the slot in the second flange thereby enhancing the conductivity of the junction between the bar and the hook. The hook is nevertheless easily removed from the bar for substituting a new and uncoated hook or installing the same hook after the hook has been cleaned.
An important feature of the present invention is the provision of a horizontal bar or beam of enhanced stiffness, which resists bending along either the x-axis or y-axis of its cross-section. Prior art horizontal bars, such as those in the patents cited above, although providing acceptable stiffness along the y-axis, provide inferior stiffness against bending along the x-axis.